This functionality is included with MindView 4. Simply export your mind map as PDF from the Export menu.
For older versions of the software you will need an emulated PDF printer such as http://www.pdf995.com/ installed in order to print to PDF files. When you have installed one such PDF printing utility you can select this utility as your printer.
If you do not wish to change your default printer you must select the proper printer from inside OpenMind. To do this either use the print command from the file menu or use the print preview feature and select your PDF printing utility.
Other utilities (free and commercial) include:
For other options please see Google, a search for “pdf print” yields a large number of usable results.